The development of photodiode arrays comprising large numbers of individual elements is becoming increasingly important in certain specific applications. For quality assurance, it is virtually essential that screening of a fabricated photodiode array on an individual element basis be performed prior to incorporation of the array in utilization equipment. Heretofore, such screening has been performed by mechanical probing of the elements, usually on a single element basis. The results of such a procedure are less than satisfactory. Delicate indium bumps which are provided for circuit test contacts are easily damaged by a contacting probe. The procedure is a very time-consuming operation. A further disadvantage is that the results of the procedure are extremely operator dependent.
Given the nature of the problem--small detector size (approximating 1.5.times.1.5 mils) of typical multi-element hybrid focal plane arrays and the delicate character of the indium interconnects--some type of non-contacting method of diode probing is desired, preferably one which can be automated or at least performed in a way which provides results which are independent of operator parameters.
A system has been developed which uses an electron beam integrated circuit tester for testing the internal nodes of a complex integrated circuit. This system incorporates electron beam apparatus which focuses and directs the electron beam to selected internal nodes and detects secondary electron emission therefrom with associated computerized control circuitry. As thus controlled, the electron beam apparatus provides high speed access and testing of the integrated circuit nodes.
In one version of the system, the probe intelligently chooses a limited number of the internal nodes which are considered most likely to indicate circuit failure so as to minimize the number of nodes tested while maximizing reliability of the results. The probe comprises an artificial intelligence which understands the design and operation of the integrated circuit under test, the intelligence being embodied in a programmed computer associated with the probe. The system further includes circuitry for accessing the peripheral pads of the integrated circuit under test and for applying the proper circuit biases, clock signals and test signals under control of the computer. In such a system, the movement of the electron beam between selected internal nodes of the circuit and the operation of the associated computer to select those nodes are decisions which may be made in a matter of microseconds or milliseconds.
The electron beam of the test apparatus, when focused on a single selected node within the circuit, creates secondary electron emission having a flux which is affected by the properties of the node on which the electrons of the primary beam impinge. A suitable detector responds to the secondary emission electron flux, permitting the computer to sense and store the voltage of the node under test.
A particular advantage of such a system is that the probe does not capacitively load any of the internal nodes of the integrated circuit under test, nor does it damage them. At the same time, the electron beam of the probe has a submicrometer diameter and is easy to position with great precision. Because of the high speed with which the nodes may be selected and tested, use of the system on a production line having high product through-put is cost effective and is inherently more reliable than the conventional testing methods outlined hereinabove.
E-beam testing of individual photodiodes in a photodiode array has also been accomplished. Instead of directing the electron beam to internal circuit nodes, as described above, the beam is used to access the free electrode of a selected diode in the array. These arrays comprise individual photodiodes, one electrode of which is connected to a common ground or reference plane. The other electrodes are individually coupled to corresponding indium antimonide bumps which complete the circuit to the associated photodiodes when the diode array is mounted in utilizing apparatus. These other electrodes are also wire bonded in selected groups to an external pin connection.
In testing the photodiode array, the E-beam is used to direct the electrons to the selected diode, developing a predetermined voltage thereon. The corresponding current of the diode is measured by an electrometer coupled in the circuit path to the common diode substrate reference plane. Various diode voltage levels are established by controllably varying the duty cycle of the pulsed E-beam and are measured by an associated voltage contrast sensor comprising a scintillator which responds to secondary emission electron flux from the particular diode under test. The particular I-V measurements developed in the manner described are compared with corresponding I-V plots of similar diodes of known quality characteristics, utilized as a reference, to determine if the diode under test is acceptable or not.
The reliability of the I-V measurements developed in this manner depends upon an assumption that the quality characteristics of the diode under test do not differ materially from those of the diode used as a reference. This is intended to be a reasonable assumption, since the diode used as a reference will be one which is located on the same wafer as the diode array under test, thus presumably having been fabricated under the same conditions. It does not always work out that way, however. Sometimes, for reasons which are not fully understood, there may be substantial variations in the operating parameters and characteristics of different diodes on the same wafer.
Additionally, the I-V measurements depend upon an indirect calibration of the voltage contrast sensor. The voltage levels measured by the voltage contrast sensor require resort to a look-up table, derived from an indirect calibration of the voltage contrast sensor, to determine the voltage levels of the I-V measurements taken for the diode undergoing test.
It would be preferable to calibrate the voltage contrast sensor from the actual diode under test. However, the typical low background curve for the test diode does not admit of precise determinations of the true values for different voltage levels. It would be desirable to be able to calibrate the voltage contrast sensor for an individual diode by measuring the zero-current crossings of the voltage axis with increased precision, sufficient to make the calibration reliable.